1. Field of the Invention
This invention relates to a fuel injection system for the internal combustion engine, and, more particularly, it is concerned with the fuel injection system using an air flow sensor (hereinafter abbreviated as "AFS") for detecting an air flow rate in both forward and backward directions.
2. Discussion of Background
In the following, a conventional fuel injection system for the internal combustion engine will be explained in conjunction with FIGS. 1 and 2 of the accompanying drawing which illustrate one embodiment of fuel injection system according to the present invention.
In FIG. 1, a reference numeral 1 designates an internal combustion engine which is mounted on an automobile, etc., and in which only one cylinder is shown out of a plurality of cylinders; a numeral 2 refers to a cylinder of the internal combustion engine 1; a numeral 3 refers to an air intake valve to be driven by a cam (not shown in the drawing); a reference numeral 4 designates an intake manifold of the internal combustion engine 1; a numeral 5 denotes a surge tank which is connected to the upstream side of the intake manifold 4; a numeral 6 refers to an intake air temperature sensor for detecting temperature of the intake air; a reference numeral 7 designates a throttle valve provided in the air inlet passage which is at the upstream of the surge tank 5 and for controlling the intake air quantity into the internal combustion engine 1; a reference numeral 8 denotes a sensor connected to the throttle valve 7 and for detecting a degree of opening of the throttle valve; a numeral 9 refers to a bypass which functions to detour both upstream and downstream of the throttle valve 7; a numeral 10 refers to a bypass air quantity regulator provided in the bypass 9; a numeral 11 refers to a heat-wire type AFS which is provided at a location further upstream of the throttle valve 7 and for detecting an air quantity to be taken into the internal combustion engine 1 by means of, for example, a temperature-dependent-resistor; a reference numeral 12 designates an air cleaner provided at an intake port situated at the upstream of the AFS 11; a reference numeral 13 represents a fuel-injection valve for feeding by injection the fuel into the internal combustion engine, the fuel-injection valve being provided in the intake manifold 4 for each and every cylinder 2; a reference numeral 14 designates a water temperature sensor for detecting temperature of the cooling water in the internal combustion engine 1; a numeral 15 refers to a crank angle sensor for detecting a predetermined crank angle of the internal combustion engine 1; a numeral 16 refers to a starter switch; a reference numeral 17 designates a neutral detection switch; a reference numeral 18 designates an electronic control unit (hereinafter abbreviated as "ECU") for controlling the fuel injection quantity from the fuel injection valve 13 to take a predetermined air/fuel proportion with respect to the air quantity to be taken into each of the cylinders of the internal combustion engine 1, the ECU functioning to determine the fuel injection quantity based principally on those signals from the AFS 11, the water temperature sensor 14, the crank angle sensor 15 and the starter switch 16 to thereby control a fuel injection pulse width in synchronism with the signal from the crank angle sensor 15.
In the following, the detailed construction of the above-mentioned ECU will be explained. Referring first to FIG. 2 of the accompanying drawing, a reference numeral 18a designates a digital interface for introducing input digital signals from the crank angle sensor 15, the starter switch 16, the neutral detection switch 17, and so on. This digital interface 18a is connected to an input port or an interruption terminal of a CPU (central processing unit) 18e. A reference numeral 18b designates an analog interface for introducing input analog signals from the intake air temperature sensor 6, the throttle valve opening degree sensor 8, the AFS 11, the water temperature sensor 14, and so forth. The outputs from this analog interface 18c are sequentially selected by a multiplexer 18c, subjected to the digital/analog conversion by means of an A/D converter 18d, and taken into the CPU as the digital values. The CPU 18e is a well known mico-processor comprising control programs, data-inscribed ROM, and a timer, and generates by means of a timer output a fuel injection pulse width which is computed by the predetermined control programs. A reference numeral 18f denotes a drive circuit which is for driving the fuel injection valve 13 with the above-mentioned pulse width.
FIG. 11 is a block diagram for explaining in further details the conventional operations of the above-mentioned CPU 18e. In the drawing, a reference numeral 181 designates an engine-revolution detecting section which converts a cycle of square wave signals generated from the crank angle sensor 15 into the number of revolution of the internal combustion engine; a reference numeral 182 denotes an average air quantity detecting section for finding out an average air quantity by converting the voltage of the AFS 11 into an air flow rate, and averaging the thus converted flow rate between the signals from the crank angle sensor; a numeral 183 refers to an air quantity limiter, which is constructed with a maximum air quantity computing section 183a for finding out the maximum air quantity in a reference atmospheric condition as established in correspondence to the number of revolution of the internal combustion engine and a limiting section 183b for limiting the upper part of an output from the average air quantity detecting section 182 with the output from the maximum air quantity computing section 183a; a reference numeral 184 designates a charging efficiency calculating section for finding out a charging efficiency (.eta.) by dividing an output from the air quantity limiter 183 by an output from the engine-revolution detecting section 181, the dividend of which is multiplied by a predetermined coefficient; and a numeral 185 refers to an injection pulse width computing section for finding a time width of a pulse for the fuel injection quantity by multiplying an output from a warming-up load calculating section 186 which generates a loading coefficient (C.sub.wt) in accordance with an output from the water temperature sensor 14 and the above-mentioned charging efficiency (.eta.), and then by further multiplying a discharge quantity coefficient (R) of the fuel injection valve 13.
In the foregoing, the construction of the conventional fuel injection device for the internal combustion engine has been described in detail. Now in the following, particular explanations will be given as to the necessity for providing the air quantity limiter 183 shown in FIG. 11.
For the fuel control of the internal combustion engine 1, there is effected detection by the AFS 11 of an air quantity which is supplied from the air cleaner 12 into the surge tank 5 by way of the intake manifold 4, as shown in FIG. 1, and then detection of the temperature of the intake air by the intake air temperature sensor 6. In case, however, of using the AFS 11 for the automobile, etc., there may possibly take place reversal in the flow of the air.
Such reversed flow may become considerable in most cases when the throttle valve 7 is in its full open condition with a number of revolution of the internal combustion engine ranging from 1,000 to 3,000 rpm. For the sake of simplicity, this reversed flow of air will hereinafter be termed as "back-flow". When this back-flow occurs, the AFS 11 would detect in principle the quantity of even such back-flow air, owing to which the AFS effects excessive measurement of the air quantity which is taken into the cylinder 2 of the internal combustion engine 1. Further, this measured value reaches, in some cases, from 1.5 to 2 times as large as the normal value, and, in the absence of appropriate measured being taken, the feeding quantity of fuel into the internal combustion engine 1 becomes excessive. In order therefore to avoid such erroneous and excessive injection of fuel from the fuel injection valve 13, the air quantity limiter 183 is provided. This air quantity limiter 183 functions to avoid the excessive supply of the fuel through the mis-calculation done by the above-mentioned AFS 11 by first finding out a real value of the intake air quantity for the internal combustion engine 1 under the reference conditions of the atmospheric pressure and temperature in accordance with the number of engine-revolution, storing this value of the intake air quantity as the mapping data for the number of revolution of the engine, and limiting an output from the average air quantity detecting section 182 based on the mapping data for the number of revolution of the engine.
Since the conventional fuel injection device of the internal combustion engine is constructed as mentioned above, when, for example, an automobile is driven at a high elevation, the air quantity limiter 183 is not capable of controlling the air quantity to an appropriate limit value in correspondence to reduction in the atmospheric pressure. On account of this, there occurs various problems such that an excessive quantity of fuel is supplied to the internal combustion engine 1 during the vehicle driving with the throttle valve 7 being in full-open condition at a low engine revolution, and others. The reason for this is that, at a high elevation of 3,000 m above the sea level, for instance, the atmospheric pressure becomes as low as 530 mmHg, owing to which the fuel is supplied in excess of approximately 30% with the full-open throttle valve, thereby causing disorder in the internal combustion engine 1. While this problem may be solved by use of an atmospheric pressure sensor, there is a new problem of increased cost for its installation.